This invention relates to a radio communication system which uses an adaptive modulation and coding method to achieve high-speed data transmission.
An adaptive modulation and coding rate communication method for a communication system is generally known wherein the coding rate and the multi-value modulation factor of error correction codes are varied in response to the propagation path quality such that, to a user to whom the propagation quality is high, high-speed data communication is provided while the noise resisting property is sacrificed, but to another user to whom the propagation quality is low, low-speed data communication is provided attaching importance to the noise resisting property. One of such adaptive modulation and coding rate communication methods is disclosed, for example, in Japanese Patent Laid-Open No. 2003-174485.
In recent years, a communication method which uses such adaptive modulation as described above has been adapted also to a radio communication system which involves mobile radio communication for a portable telephone system and so forth. One of such radio communication systems is the HDR (High Data Rate). Also the W-CDMA (Wide-band Code Division Multiple Access) additionally adopts a similar method (HSDPA: High Speed Downlink Packet Access).
The communication method described achieves communication which adopts the adaptive modulation-coding rate in accordance with the following basic procedure.    1. A terminal measures a channel quality of a signal transmitted thereto from a base station.    2. The terminal notifies the base station of a modulation method and coding rate (hereinafter referred to as mode requesting message) estimated to be optimum from a result of the channel quality measurement.    3. The base station determines a modulation system-coding rate (hereinafter referred to generally as transmission mode) to be allocated actually from the mode requesting message received from the terminal and a state of resources of the base state and transmits a parameter (transmission parameter) of the determined transmission mode to the terminal.    4. The base state transmits user data based on the determined transmission parameter.    5. The terminal receives the transmission parameter and performs a data reception process based on the transmission parameter.    6. If an error is detected in the received data, then the terminal returns a re-sending request, but if the data is received accurately, then the terminal transmits a new data transmission request to the base station.    7. The steps 1 to 6 above are repeated cyclically.
The processing procedure described above is illustrated in FIG. 4. FIG. 4 particularly illustrates a relationship among a down control channel for notifying the terminal of the transmission parameter of the down data transmission from the base station, a down data channel for transmitting user data from the base station and a control channel for transmitting the transmission parameter request from the terminal. FIG. 4 illustrates an example wherein the steps 1 to 6 described above are repeated in a frame cycle.
When the base station performs transmission of down data, it varies the data transmission rate in accordance with a reception situation (channel quality) of the user terminal so that it transmits data with a higher efficiency to the user terminal side. Further, the base station attaches importance to the efficiency of the system and allocates a predetermined data transmission radio source to a user terminal which has a channel quality relatively higher than a long term average channel quality.
FIG. 5 shows an example of a configuration of a conventional base station which implements the communication method described above. Referring to FIG. 5, the base station includes a transmission/reception apparatus 1101, a despreading section 1102, a channel quality message extraction section 1105, a mode decision section 1106, a mode control section 1107, a spreading section 1111, and an adaptive coding modulation section 1112.
The base station demodulates a transmission signal from a user by means of the transmission/reception apparatus 1101 and the despreading section 1102. The base state acquires a channel quality value transmitted thereto from a terminal from the demodulated data by means of the channel quality message extraction section 1105. The mode decision section 1106 selects an optimum transmission mode (modulation and coding methods) from the extracted channel quality. The mode control section 1107 performs setting of the adaptive coding modulation section 1112 in accordance with the transmission mode decided by the mode decision section 1106 to control the user data channel. The modulated and coded user data and a pilot signal to be used for synchronization are spread by the spreading section 1111. At this time, the spreading factor of the user data and the pilot signal spreading factor do not necessarily coincide with each other, and they have a relationship of the user data spreading factor SFd<=pilot signal spreading factor SFp. The spread combined signal is transmitted as a radio signal through the transmission/reception apparatus 1101.
FIG. 6 shows an example of a configuration of a conventional user terminal which implements the communication method described above.
Referring to FIG. 6, the user terminal includes a transmission/reception apparatus 1201, a despreading section 1202, a channel estimation section 1203, a synchronous detection section 1204, a demodulation-decoding section 1205, a pilot channel quality estimation section 1206, a data channel quality conversion section 1207, a channel quality message insertion section 1213 and a spreading section 1215.
A transmission signal signaled from the base station is received through the transmission/reception apparatus 1201 and separated into data of a data channel and pilot channel data by a despreading process by the despreading section 1202. The demultiplexed pilot signal is inputted to the channel estimation section 1203, by which multi-path fading environments are estimated. The channel estimation value as the multi-path fading environments can be derived in accordance with, for example, the following expression (1) utilizing known data (pilot signal):
                              h          ⋒                =                              ∑            i                    ⁢                                    r              p                        ⁡                          [              i              ]                                                          (        1        )            where rp is the despread pilot signal, and the hat mark of the variable h on the left side represents that the variable h is an estimated value.
The demultiplexed data signal is subject to compensation for a phase variation given thereto by the radio propagation path by multiplying the reception data (despread data) rd by a complex conjugate of the channel estimation value determined in accordance with the expression (1) as in the following expression (2) by means of the synchronous detection section 1204:d=ĥ*×rd  (2)where rd is the despread data.
The data d outputted from the synchronous detection section 1204 undergoes a demodulation process and an error correction process by the demodulation-decoding section 1205 to form reception data.
The pilot channel quality estimation section 1206 determines a signal S of the pilot channel in accordance with expressions (3) and (4) given below, determines noise of the signal S in accordance with an expression (5) given below and calculates a signal to noise ratio (SNR) of the pilot channel in accordance with an expression (6) given below:
                    s        =                              1            M                    ⁢                                          ⁢                                    ∑              i                        ⁢                          (                                                                    h                    ⋒                                    *                                ×                                                      r                    p                                    ⁡                                      [                    i                    ]                                                              )                                                          (        3        )                                S        =                                          s                                2                                    (        4        )                                N        =                              1                          M              -              1                                ⁢                                          ⁢                                    ∑              i                        ⁢                                                                                                                      r                      p                                        ⁡                                          [                      i                      ]                                                        -                  s                                                            2                                                          (        5        )                                          SNR          p                =                  10          ⁢                                          ⁢          log          ⁢                                          ⁢                                    S              N                        ⁢                                                  [            dB            ]                                              (        6        )            where rp is the despread pilot signal, and M is the average length (in the present example, the pilot symbol number in one slot is 10).
The data channel quality conversion 1207 converts the pilot channel quality into a data channel quality. This conversion corrects the difference between the spreading factors and is performed in accordance with the following expression (7):
                              SNR          d                =                              SNR            p                    +                      10            ⁢                                                  ⁢                          log              ⁡                              (                                                      SF                    d                                                        SF                    p                                                  )                                                                        (        7        )            where SFp is the spreading factor for the pilot signal, and SFd is the spreading factor for the data channel.
The second term on the right side of the expression (7) usually has a negative value because the user data spreading factor SFd is usually lower than the pilot signal spreading factor SFp as described hereinabove. In other words, the expression (7) functions such that a value obtained by decreasing the channel quality estimation value of the pilot channel in response to the ratio between the two spreading factors is used as an estimation value of the channel quality of the data channel. It can be considered that this compensates for noise of a high frequency portion removed by the despreading as seen from FIG. 7. In other words, the despreading can be regarded as a filter function, and it is considered that, as the spreading factor increases, the bandwidth of the filter decreases. Accordingly, the noise component obtained by decreasing the reception signal of the data channel having a lower spreading factor is greater than the noise component obtained by despreading the reception signal of the pilot channel having a higher spreading factor.
The channel quality message insertion section 1213 inserts the data channel quality value obtained in accordance with the expression (7) into the transmission data.
The spreading section 1215 spreads the transmission data and the channel quality value and transmits the resulting data as a radio signal to the base station through the transmission/reception apparatus 1201.
The user terminal having the configuration described above allows selection of a modulation and coding method in accordance with the channel quality of the terminal on the base station side. Thus, the data transmission rate can be varied in accordance with a reception situation of a user terminal to achieve efficient data transmission.
In this manner, the base station determines the data transmission rate in order to transmit data in accordance with a channel quality of the user terminal. Therefore, in order to use the system efficiently, the accuracy of the channel quality value estimated by the terminal side is significant.
On the other hand, the spreading factor conversion applied by the expression (7) is based on the assumption that the noise is white. As seen from the following expressions (8) and (9), a reception signal rx includes a desired signal s, noise nch generated in a radio section and noise no generated in the inside of the receiver, and the assumption of the expression (7) is valid in a situation wherein internal noise of the receiver can be ignored.rx=s+nch+no  (8)no=nwhite+ncolor  (9)
However, as seen in FIG. 9, under a situation wherein the SNR is extremely good, that is, under a situation wherein very high quality data transmission is possible, a state wherein the noise no cannot be ignored is established, and there is the necessity to take noise factors ncolor (colored noise) other than white noise which composes receiver noise into consideration. The noise factors ncolor are generated from a PLL (Phase Locked Loop) and an AFC (Automatic Frequency Control) which compose a radio unit, and generally, low frequency noise belonging to a low frequency region from within noise which can be observed from a pilot signal as seen from FIG. 8. Therefore, if noise estimated from a pilot signal (low-pass filtered signal) having a high spreading factor is converted into data channel noise in accordance with the expression (7), then it is estimated that also the high frequency region includes similar noise to that in the low frequency region. This causes the amount of the noise in the high frequency region to be estimated to be more than the actual noise amount, resulting in failure of enjoyment by a user of a service of a high data rate which the user can originally enjoy.